Process for polymerizing unsaturated compounds in an aqueous medium



1957 c. P. VAN DIJK ETAL PROCESS FOR POLYMERIZING UNSATURATED COMPOUNDSIN AN AQUEOUS MEDIUM Filed Jan.

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h 0 0.1 in 6 W "mi m h 1 m F Franmscus Fmdannus VonDu-Pm B5 TheirA+f0rnq= United StatesPatent PROCESS FOR POLYNIERIZHNG UNSATURATEDCOMPOUNDS IN AN AQUEOUS MEDIUM Christiaan Pieter van Dijk and FranciscusJohannes Fredericus van der Plas, Amsterdam, Netherlands, assignors toShell Development Company, Emeryville, Calif, a corporation of DelawareApplication January 7, 1952, Serial N 0. 265,302 Claims priority,application Netherlands January 17, 1951 7 Claims. (Cl. 26092.8)

This invention relates to a process for polymerizing unsaturatedcompounds, and more particularly, to an improved process forpolymerizing ethylenically unsaturated compounds in an aqueous medium.

Specifically, the invention provides a new process for polymerizingsubstantially water-insoluble ethylenically unsaturated compounds in anaqueous medium which process has improved operating efficiency andyields polymers having superior mechanical properties. The improvementin efficiency and quality of product is obtained by adding drops of theunsaturated compound to an aqeous medium which contains a polymerizationcatalyst and is being maintained at a polymerizing temperature undersuch conditions that the drops of monomer rise or fall through themedium due to the difference in density of the drops and the medium andare subsequently collected as a separate phase in contact with but aboveor below the aqueous medium, and at the same time continuouslycirculating the aqueous medium in a substantially vertical plane, themovement of the aqueous medium above or near the point of introducingthe drops preferably being in substantially the same direction as themovement of the drops of monomer.

In our copending application, Serial No. 201,536, filed December 19,1950, now U. S. Patent No. 2,618,626, there is described and claimed aprocess for polymerizing substantially water-insoluble compounds in anaqueous medium which comprises conducting drops of the compound incontact with and through a liquid medium which contains a polymerizationcatalyst and is being maintained at a polymerizing temperature. Themonomer drops that pass through the aqueous medium are then collectedinto a separate phase and transported outside the chamber filled withthe aqueous phase back to the point of supply for the drops.

The process described in our above-mentioned copending patentapplication presents many important advantages over known polymerizationmethods. The process permits, for example, better control over thereaction temperature as the heat of reaction may be removed by coolingthe externally circulated monomer rather than by cooling the walls ofthe reaction chamber itself. Secondly, the unique step of circulatingthe monomer drops through the reaction mixture avoids the necessity ofusing mechanical stirring and this in turn decreases or eliminates theformation of polymer deposits on the walls of the reaction chamber.Furthermore, the passage of monomer drops through the water providesbetter control over the concentration of monomer in the water phase.

There are, however, certain features of the process described in thecopending application that tend to reduce its operating efficiency. Ithas been found, for example, that in some instances the passage of themonomer drops from the aqueous phase to the monomer phase is rather slowand this causes a considerable reduction in the amount of polymer formedper unit time.

In addition, it has been found that there are certain locations in thereaction zone, and particularly those behind or near the device forintroducing the monomer drops, Where the saturation with the monomer isinsuflicient. The presence of these dead spaces not only tends todecrease etficiency of the process but gives rise to the formation ofproducts having rather poor mechanical properties.

It is an object of the invention, therefore, to provide an improvedmethod for polymerizing substantially water-insoluble ethylenicallyunsaturated compounds in an aqueous medium. It is a further object toprovide a process for polymerizing unsaturated compounds in an aqueousmedium that permits anincreased rate of formation of polymer. It is afurther object to provide a process for polymerizing unsaturatedcompounds by passing monomer drops through the aqueous medium whichprocess has improved means for speeding the collection of the drops in amonomer phase at the end of their passage through the medium. his afurther object to provide a process for polymerizing unsaturatedcompounds in an aqueous medium that yields products having improvedmechanical properties. It is a further object to provide a process forpreparing homogeneous copolymers in an aqueous medium. These and otherobjects .of the invention will be understood from the following detaileddescription thereof and from the accompanying drawing wherein Figs. I toVI are diagrammatic representations of various apparatus that may beused in carrying out the process of the invention.

It has now been discovered that these and other objects are accomplishedby the process of the present invention which comprises adding drops ofthe substantially waterinsoluble ethylenically unsaturated compounds toan aqueous medium containing a polymerization catalyst a polymerizingtemperatureand is being maintained at under such conditions that thedrops of monomer rise or fall through the medium due primarily to thediffere'nce in density of the drops and the medium and the said dropssubsequently collected as a separate phase in contact with but above orbelow the aqueous medium, and at the same time, continuously circulatingthe aqueous medium in a substantially vertical plane, the movement ofthe aqueous medium ducing the drops of monomer preferably being insubstantially the same direction as the movement of the said monomerdrops. the aqueous phase in the above-described manner one can effect avery rapid increase in the rate of movement of the monomer drops fromthe water into the monomer layer. The increased rate of passage into themonomer layer is particularly apparent where there is a tendency to forman emulsified layer at the interface. In these cases, the movement ofthe water phase removes the emulsified layer along the interface andleaves the said interface relatively clean for the penetration of themonomer drops. It has also been surprisingly found that by circulatingthe aqueous phase in the direction of the movement of the monomer drops,the monomer drops can be introduced at a much faster rate than waspossible heretofore without having the liquid monomer dispersed into toofine drops. As a result of these features, the polymer can be formed inthe aqueous medium at a greatly improved rate.

It hasbeen further found thatthe circulation of the aqueous phase in theabove-described manner eliminates the dead spaces reaction medium hassubstantially the same degree of monomer saturation. The polymers formedby this process, therefore, have more uniform properties and can be usedto prepare products having improved strength and flexibility.

above or near the point of intro-- It has been found that by circulatingWithin the reaction zone and the entire The circulation of the aqueousphase in the substantially vertical plane may be accomplished in a greatvariety of different ways. It is preferably accomplished by maintainingat least a part of the space occupied by the aqueous medium as a closedcircuit wherein the aqueous phase is allowed to circulate. This ispreferably accomplished by employing a reaction chamber having one ormore hollow or solid fixed barriers secured to and between any twoopposite walls of the chamber whereby the aqueous phase can flow upbetween the barrier and the vessel wall, over the barrier, and then backbetween the barrier and the vessel wall to the point of beginning. Thebarrier or barriers may take any shape or size and may occupy any lengthof the vessel as long as the proper amount of aqueous phase is permittedto pass over and under the said barrier or barriers.

A few examples of suitable reaction vessels containing a fixed barrierare shown in Figs. I, II, III, IV and VI of the attached drawing. Fig. Irepresents a reaction vessel having a short fixed solid barrier in theupper part of the reaction vessel, and Fig. II represents a reactionvessel having a short fixed solid barrier in the lower part of thevessel. Figs. III and VI represent suitable vessels having a fixed solidbarrier running through a greater part of the vessel. Fig. VI differsfrom Fig. III in having the distributing device at the top (for monomersheavier than water). Fig. IV represents a similar type of reactionvessel having a streamlined fixed barrier.

Fig. V represents a suitable reaction vessel having two fixed solidbarriers attached to two opposite sides of the vessel. In this case, thedistributing devices will be located in the two outer compartments andthe aqueous phase will rise through the outer compartments and returnthrough the inner compartment. Reaction vessels of this type may also beobtained by placing an open cylinder in the center of the reactionvessel and having the distributing device fitted annularly into theouter compartment. In this case, the water phase will rise through theouter compartment and return through the inner cylinder.

The exact size and shape of the reaction vessel may vary over a widerange depending chiefly on the desired advantages to be accentuated. Forexample, if it is desired to have a particularly superior rate ofseparation of the monomer drops into the monomer phase, the vesselshould be so constructed as to have the area of the interface betweenthe aqueous phase and the monomer phase in a large cross-section area.In addition, the vessel should be so constructed as to permit theaqueous phase to be circulated past all or a considerable part of theinterface between the monomer layer and the aqueous phase. As indicatedabove, this permits the aqueous phase to remove the contaminatingparticles at the interface and allows a more rapid penetration of theinterface by the monomer drops. An example of a reaction vesselpossessing these features is shown in Fig. I. In the apparatus shown inthis figure, the largest cross-sectional area is at the top where theinterface between monomer layer 3 and aqueous phase 1 is located and theaqueous phase circulates upwards in the direction of the arrow andpasses along the interface and then returns back to the point ofbeginning.

If one desires to eliminate all of the dead spaces in the reaction zone,it is also desirable to have the vessel constructed so as to have thedevice for introducing the monomer drops in the aqueous phase in thepath of the circulating aqueous phase. An example of this type ofreaction .vesselis shown in Fig. II. In the apparatus shown in thisfigure, the aqueous phase circulates upwards in the direction of thearrow around-the barrier and then back to and around distributing device2.

The most superior resultsare obtained, of course, by employing vesselspossessing all of the above-noted features, i. e., by having a largecross-section area near the location of the interface, having thehorizontal part 4 of the circuit of the aqueous phase pass along theinterface, and by having the distributing device located in the path ofthe circulating aqueous phase. Figs. III to VI illustrate suitablevessels of this type. In Figs. III, IV and VI, the aqueous phasecirculates by the distributing device 2, thence upwards or downwards asthe case may be to the interface between the monomer phase and theaqueous phase, along the interface and thence back to the beginning. InFig. V, the aqueous phase circulates by the distributing device 2 fittedannularly in the outer compartment, thence upwards in the outercompartment, along part of the interface and thence back through theinner compartment.

The circulation of the water phase may be brought about by the use of apump but it is generally desirable to effect the circulation by means ofthe impulse caused by the movement of the monomer drops through the saidaqueous phase. By introducing the monomer drops at a relatively highrate, one can obtain a very satisfactory rate of circulation of theaqueous phase. Care should be taken, however, not to utilize anexcessive rate of introducing the monomer drops as such rates tend todecrease the size of the drops and impair the uniformity of the drops.This, of course, has a detrimental effect on the separation of the dropsinto the monomer phase. In general, the linear velocity of the monomerdrops should not be more than 30 cm./ sec. in excess of the velocity ofthe aqueous phase, and more preferably not more than 8 to 12 cm./sec. inexcess of the velocity of the aqueous phase.

In the operation of the process of the invention, the reaction vessel isfilled with the aqueous phase and then the unsaturated monomer passedinto the aqueous phase in the form of drops and those that completelypass through the phase are then collected as a separate phase in contactwith the aqueous medium. If the monomer is lighter than water, it willbe introduced at the bottom of the vessel and will pass upwards throughthe aqueous medium, while if the monomer is heavier than the aqueousphase, the monomer will be introduced at the top and pass downwardsthrough the aqueous medium.

The term drops ofmonomer as used throughout the specification and claimsmeans liquid globules of monomer that are of such size that they areclearly visible as distinct entities to the unaided human eye and arethus distinguishable from monomer droplets present in the aqueousemulsion processes. The droplets in the emulsion are so small as to besubjected to the Brownian move ment, while the drops of monomer in thepresent invention are so large as to be unaffected by this movement andmove through the aqueous medium only by floating or sinking realizedfrom the difference in the density of the drops and the aqueous medium.The drops preferably have a diameter of about 0.1 to 2 centimeters andmore preferably from 0.1 to l centimeter.

The drops of the unsaturated monomer are preferably added to the aqueousmedium through a distributing device containing a plurality of aperturesthrough which the drops may enter the medium. The entrance aperture orapertures can have a diameter of say 0.2 to 20 mm. and are usually 2 to10 mm. The entering means is placed at one end of the reaction chamberso that the drops flow away from it. At the vertically opposite end, thedrops which have passed through the aqueous medium are collected into aseparate phase in contact with the aqueous phase.

The amount of the aqueous medium through which the drops of monomer areallowed to rise or fall may vary over a wide range. In most cases, themedium is so regulated as to permit a recovery of a great part of thedrops in a separate phase at the opposite end of the reaction chamber.As the monomer is being consumed in the polymerization reaction in theaqueous medium, there will be some diffusion, e. g., about 0.1% to about10%, of the monomer into the medium per pass and the remaining part ofthe monomer will be recovered in each pass.

The monomer drops that pass through the aqueous medium are collected asa separate integral phase and then as further drops are collected aportion of that phase is withdrawn and recycled back to the point ofsupply of the drops.

The recycling of the monomer may be accomplished in any suitable mannerbut is preferably accomplished by means of an outside conduit so thatthe monomer may be cooled before it is introduced into the reactionzone. As indicated above, this cooling of the recycled monomer outsidethe reaction chamber offers a splendid opportunity for heat control ofthe polymerization reaction.

It is also advantageous in many cases to subject the circulating monomerto a washing operation before returning it to the polymerization zone.For this purpose, washing water gives good results and is easilyeffected by bringing the drops into contact with a column of water,conducting the drops through the water, and then collecting the passeddrops into an integral layer for transportation to the reactor.

After some polymer, e. g., about to and more preferably from 10% to hasbeen formed, portions of the polymer-containing aqueous phase areremoved from the reaction chamber. The removal of the aqueous phase isaccomplished by withdrawing portions of the aqueous medium, preferablyat about the rate at which the polymer is being formed, and then addingwater, catalyst and other necessary components to make up for thatwithdrawn. The concentration of catalyst and other components arepreferably maintained substantially constant in the aqueous phase. Thisis conveniently accomplished by using as feed stock a mixture of water,catalyst, etc., having the components in the desired pro portions. Ifdesired, however, one or all of the components may be added separatelyfrom the aqueous feed.

In removing portions of the aqueous phase, the best results are obtainedby having the place or places of withdrawal located in a region free ofmoving monomer drops such as in the vertical side of the circuitcarrying the aqueous phase away from the interface of monomer phase andwater phase. If the distributing device is not located in the path ofthe circulating aqueous phase, it is generally desirable to remove thepolymer-containing aqueous phase at positions located behind thedistributing device. If the distributing device is located in the pathof the circulating aqueous phase, the polymer-containing aqueous phaseis preferably removed at positions just before the circulating phasereaches the distributing devrce.

The process of the invention may be better understood by considering howthe process operates in several of the reaction vessels shown in theattached drawing. In the operation of the vessel shown in Figure I, thereaction vessel is partly filled with the desired aqueous phase 1 andthen the monomer drops introduced at the bottom through distributingdevice 2. The monomer drops rise through the aqueous phase and thosethat complete the passage are collected in a monomer layer 3 at the top.The impulse of the monomer drops causes the aqueous phase to circulateupwards in the direction of the arrow. The water phase then passes bythe interface between the monomer layer and the aqueous phase,returnsdown the other side and thence back to the middle of the oppositeside where it is again carried upwards with the monomer drops. A portionof the monomer layer is recycled through line 4 by means of pump 5 todistributing device 2 at the bottom. Fresh monomer can be suppliedthrough line 8 and the polymer-containing water phase can be dischargedthrough line 9 with valve 10.

In the operation of the vessel shown in Fig. VI, the reaction vessel ispartly filled with the desired aqueous phase 1 and then monomer dropsintroduced at the top through distributing device 2. The monomer dropsthen fall through the aqueous phase and are collected as a separatephase 3 at the bottom of the reaction vessel. The impulse of the monomerdrops causes the aqueous phase to circulate downwards in the directionof the arrow. The water phase then passes by the interface between themonomer layer and the aqueous phase, returns up the other side and thenback past the distributing device. A portion of the monomer layer isrecycled through line 4 by means of pump 5 to distributing device 2 atthe top. Fresh monomer can be supplied through line 8 and thepolymer-containing water phase can be discharged through line 8 and thepolymer-containing water phase can be discharged through line 9 withvalve 10.

The monomers to be polymerized or copolymerized by the process of theinvention comprise the substantially water-insoluble ethylenicallyunsaturated organic com pounds. The expression substantiallywater-insoluble as used throughout the specification and claims inrelation to the monomer to be polymerized, refers to those monomerswhich have at least some solubility in water so that some of the monomermay enter the aqueous medium but still have so little solubility inwater that they are regarded as being relatively water-insoluble.Preferably, the monomers have a solubility in water at room temperatureof from 0.1 part to 20 parts per parts of water. Particularly preferredmonomers are those having a solubility of from 0.1 part to 14 parts per100 parts by weight of water.

The expression ethylenically unsaturated, as used throughout thespecification and claims, refers to those monomers possessing one ormore polymerizable ethylenic groups in their molecule. Examples of suchmonomers include ethylene, m-aleic acid esters, tetrahaloethylene,

butadiene-l,3, dirnethyl-butadiene-l,3, piperylene, isoprene,chloroprene, styrene, alpha-methy1 styrene, dichlorostyrene, vinylphenol; esters of unsaturated acids, such as methyl acrylate, butyl'acrylate, cyclohexyl 3-butenoate, hexyl acrylate, octyl acrylate,methyl methacrylate, butyl methacrylate, and propyl acrylate; thevinylidene halides such as vinylidene chloride and vinylidene bromide;the

vinyl esters of inorganic acids such as vinyl chloride and vinylbromide; the unsaturated nitriles, such as methacrylonitrile andacrylonitrile; the vinyl esters of monocarboxylic acids, such as vinylacetate, vinyl caproate, vinyl chloroacetate, vinyl benzoate, and vinylvalerate; the vinyl esters of polycarboxylic acids, such as divinylsuccinate, divinyl a-dipate, vinyl allyl phthalate, diallyl phthalate;the vinyl esters of the unsaturated acids, such as vinyl acrylate, vinylcrotonate, and vinyl methacrylate; the vinyl ethers, such as vinyl ethylether, vinyl butyl ether; and vinyl ketones, such as vinyl hexyl ketone,and vinyl octyl ketone.

Preferred monomers to be polymerized or copolymerized by the process ofthe invention comprise the vinylidene monomers containing apolymerizable CH2=C= group and no other polymerizable group, such asvinyl chloride, vinylidene chloride, vinyl butyrate, ethyl acrylate,styrene, methyl-styrene, allyl acetate, allyl butyrate, acrylonitrile,methacrylonitrile, isobutylene, and the like. Especially preferred arethose monomers having a molecular weight below about 225.

Of special interest, particularly because of the fine quality of thepolymers that may be prepared therefrom by the process of the invention,comprise the members of the group consisting of ethylene, vinylchloride, vinylidene chloride, butadiene, styrene, alpha-methyl styrene,

acrylonitrile, methacrylonitrile, the alkyl esters of acrylic acid, andthe vinyl esters of the alkanoic acids.

The composition of the aqueous medium to which the monomer is added mayvary depending chiefly on the,

form in which the resulting polymer is desired. It the polymer isdesired in the form of a stable emulsion, the medium may contain anemulsifying agent catalyst. 'If the mixture is desired as a dispersionfrom which the polymer may later settle out, the medium may and asuitable.

contain a water-soluble catalyst and small quantities of dispersingagents.

Catalysts used for the emulsion polymerization may be any of thepolymerization catalysts that are known to be used for this purpose,such as persulfuric acid, peracetic acid, percarbonic acid,perphosphoric acid, perphthalic acid, the persalts, such as potassiumpersulfate, the peresters, such as 0,0-tert-butyl O-et-hylmonopermalonate, the peroxides, such as hydrogen peroxide, and the like.Other catalysts can be used, such as benzoyl peroxide, tertiary butylperbenzoate, acetyl benzoyl peroxide, lauryl peroxide, acetone peroxide,etc., if they form part of the well known redox systems ofpolymerization catalyzers, especially when operating at below about 35C. Suitable catalyst systems are also combinations of oxygen and saltsof sulfurous acid. The water-soluble polymerization catalysts, and morepreferably the water-soluble peroxide polymerization catalysts, are themore desirable catalysts to be used for this type of process.

The preferred catalysts to be used for the suspensiontype polymerizationreaction include the above-described water-soluble catalysts, such aspersulfuric acid, peracetic acid, percarbonic acid, perphosphoric acid,potassium persulfate, hydrogen peroxide, and the like, and mixturesthereof.

The amount of the above-described catalysts to be used will vary over awide range depending upon their type and desired rate of polymerization.In most instances, the amount of catalyst will vary from 0.1% to andmore preferably from 0.1% to 1%, wherein the percentages are by weightbased on the aqueous phase.

Dispersing agents that may be used in the process may be exemplified bythe following: finely divided clay, talc, barium sulfate, and tricalciumphosphate, methyl cellulose, polyfluoroalkanoic acids, such asdodecafluoroheptaonic acid, pentadecafiuorooctanoic acid, salts of theseacids with saturated alkylamines, phosphoric acid esters ofpolyfluoroalkanols, and the like, and mixtures thereof.

Emulsifying agents used in the aqueous medium may be any of the knownionic or non-ionic type emulsifying materials. Suitable materialsinclude sodium and/or potassium myristate, laurate, palmitate, oleate,stearate, rosinate and/or hydroabietate; or alkali metal alkyl oralkylene sulphates, or sulphonates, such. as sodium and/ or potassiumlauryl sulfate, cetyl sulfate, oleyl sulphonate, stearyl sulphonate,sulphonated Turkey red oil, sulphonated mineral oils, etc., as well asammonium or ethanolamine salts thereof; salts of higher amines andnon-ionic emulsifiers, such as described in U. S. 2,322,820. In allcases, it is preferred that the hydrocarbon radical of the emulsifyingagent contains to carbon atoms. The amount of the emulsifying agentsused may vary over a wide range. Best results are obtained whensufficient amount is present in the aqueous medium that the suspensionof polymer is substantially stable. In general, the concentration ofemulsifying agent falls within the range of about 0.05 to 2% of theaqueous medium. With an ion-active emulsifier, there is employed ingeneral between 0.01 to 0.2, and more particularly between 0.01 and 0.06gram equivalents of emulsifier per kilogram of polymer ultimatelypresent in the dispersion. A gram equivalent is the number of gramswhich is equal to the molecular weight divided by the number of positive(or negative) elementary electric charges formed on ionization of amolecule. Thus, in using sodium cetyl sulphonate as emulsifying agentfor production of an aqueous dispersion containing 20% polymer, there isused a starting aqueous medium containing about 0.08 to 1.6%,particularly 0.16 to 0.5% of the emulsifying agent.

Although the temperature of operation in the polymerizing zone can varyduring the course of the polymerization, it is preferred to maintain itsubstantially constant. This is conveniently accomplished by cooling thecirculating monomer phase. However, in starting up the polymerization,it may be necessary to apply heat until the polymerization reaction isunder way. Once polymerization has started, the temperature is keptconstant by cooling, since the polymerization reaction is exothermic. Abroad range of temperature is suitable for conducting thepolymerization, but in general temperatures of about 20 C. to C. areused, and very good results are obtained with the aqueous medium atabout 15 C. to 70 C.

Since the process operates with liquid monomer, at least sufficientpressure is employed to achieve this state. In cases where the operatingtemperature is below the boiling point of the monomer, ordinaryatmospheric pressure may be used. With operations at temperatures abovethe boiling point, it is, of course, necessary that sufficientsuperatmospheric pressure be used that the monomer is liquid, as is thecase, for example, in polymerizing vinyl chloride at 25 C. Higherpressures, e. g., from 1 to 5 atmospheres above that needed to keep themonomer in the liquid state, may be used if desired.

The presence of oxygen generally tends to inhibit the rate ofpolymerization and, therefore, the reaction medium is preferably keptout of contact with oxygen by use of a closed apparatus and the reactoris purged free of oxygen in starting up.

The method of recovering the polymer will vary depending on the type ofmedium used. If the polymer is formed as a stable emulsion, the polymerparticles may be recovered by any coagulating means, such as freezing oraddition of coagulating agents. If the polymer is formed as a lessstable dispersion, the particles may be recovered by filtration,extraction, and the like.

Apart from the preparation of homopolymers, the process of the inventionis also very suitable for copolymerization. By this is meant the jointpolymerization of two or more monomers. The monomers used for thepreparation of the copolymers may be any mixture of the abovedescribedsubstantially water-insoluble monomers. The monomers may be combined inany desired ratio, but, in general, the mixtures of monomers areemployed wherein at least 1%, and more preferably 5%, of each is presentin the drops introduced into the water phase.

The process is particularly adapted for use in preparing copolymers ofthe homogeneous type, i. e., copolymers consisting of macromoleculeswhich contain the monomer molecules in the same ratio. In generaldifferent monomers, though they are present in the same concentration,are used up at different speeds in a copolymerization. The ratio inwhich monomers are present in the reaction consequently shows a tendencyto change. An alteration in the ratio of the monomer concentration inthe reaction mixture in turn results in the composition of themacromolecules formed being subject to alteration, so that the copolymerbecomes What is called heterogeneous.

It is known that in order to obtain homogeneous copolymer, the ratio ofthe monomers in the reaction mixture must be kept constant, which canbest be attained by admixing suitable quantities of all participatingmonomers, or by admixing suitable quantities of all monomers with theexception of the monomer which in proportion to the total quantity ofthat particular monomer present, is consumed most slowly. In the processaccording to the invention, this admixture can take place mostefficiently in the monomer circulation line. Preferably a reservoir.will be fitted in the circulation line in this case as shown in theattached drawing. The admixture of monomer then takes place preferablyin this reservoir or in the part of the circulation line in front of thereservoir. When applying a reservoir the monomer can be admixed bothcontinuously and intermittently.

The following examples are given for the purpose of illustrating theinventive improvement, but it is to be understood that the invention isnot to be construed as limited to details described therein.

9 Example I The apparatus used'in this experiment was similar to thatshown in Fig. .VI. It was made of a glass tube with a circularcrosssection and a diameter of 4 cm. The vertical parts of the circuitwere 60 cm., the horizontal parts 20 cm. in length. The corners wererounded. The distributing device for the monomer consisted of a singleglass tube pointing downwards with an inner diameter of 6 mm. Inaddition to the pump, a reservoir for the monomer was incorporated inthe connecting line between the bottom of the apparatus and thedistributing device.

The lowest horizontal part of the apparatus was filled halfway withvinylidene chloride. The remaining space in the circuit was filled withwater containing 0.5% of a mixture of sodium alkyl sulphonates (sodiummersolate), 0.3% of potassium persulphate, 0.15% of sodium bisulphiteand 0.15% sodium bicarbonate. The reservoir in the connecting line wasalso filled with vinylidene chloride. The temperature was raised to 28C. and the circulating pump was set in motion.

The circulation of the aqueous phase was elfected by the movement of themonomer drops. The vinylidene chloride was originally introduced intothe water phase at the rate of 10 liters per hour. After theconcentration of polymer in the water phase had risen to 1%, this ratewas increased to 40 liters per hour.

The distribution of the droplet size of the monomer remained practicallyhomogeneous. The separation of the droplets at the bottom of the circuitproceeded very rapidly and smoothly. The rate of polymer formation was80 parts per liter of water phase per hour. The water phase circulatedonce in 10-20 sec.

After the concentration of the polymer had risen to 20%, the process wasmade continuous. By supplying fresh water phase and discharging thepolymer suspension formed, the concentration of polymer was kept at 20%.The vinylidene chloride in the reservoir was replenished as required.

The polymer obtained by this process had excellent physical propertiesand was superior to a polymer prepared without circulating of the waterphase.

Example II This example illustrates the preparation of a homogeneouscopolymer of vinylidene chloride and ethyl acrylate using the apparatusdescribed in Example I.

The lower horizontal part of the apparatus is filled with a mixture ofvinylidene chloride and ethyl acrylate in a ratio of 80:20 by weight andthe remaining space in the circuit filled with a water phase containing0.5 ammonium persulfate and 0.1% sodium bicarbonate, and 0.5 sodiummersolate. The reservoir in the connecting line is also filled with amixture of monomers in the abovedescribed ratio.

The mixture of monomers is circulated by means of a pump and passedthrough the water phase from the top to the bottom. The temperature ofthe system is raised to 30 C. by applying heat at the heat exchanger.After the polymerization process has started, the temperature ismaintained at 30 C. by cooling with cold water in the heat exchanger.

After the concentration of copolymer in the water phase has risen to 30%by weight, fresh water phase is introduced through a conduit dischargingat a point halfway up the reactor, while the copolymer suspension formedis discharged at the top. The feed of fresh water phase is regulated insuch a manner that the concentration of the copolymer in the water phaseis maintained at 20%.

The quantity of the circulating mixture of monomers is kept up tostandard by replenishing with vinylidene and ethyl acrylate in a ratioof 80:20.

The distribution of the droplet size of the mixture of monomers remainedpractically homogeneous. The separation of the droplets at the bottom ofthe circuit proof vinylidene chloride and ethyl acrylate producedwithout circulating the aqueous phase.

Other homogeneous copolymers having improved properties are obtainedbyreplacing the vinylidene chlorideethyl acrylate in the above processwith equivalent proportions of each of the following monomer mixtures;vinylidene chloride-methyl methacrylate, vinylidenechloride-methacryloni'trile and vinylidene chloride-vinyl acetate.

Example 111 This example illustrates the homopolymerization of styreneusing an apparatus shown in Fig. III.

The apparatus used is similar to that described in Example I with theexception that the distributing device is at the bottom so that themonomer drops may pass upwards through the aqueous phase. In addition tothe pump, a reservoir for the monomer is incorporated in the connectingline between the bottom of the apparatus and the distributing device.

The lower part of the apparatus is filled with a water phase containing0.2% of sodium mersolate, 0.1% potassium persulphate, 0.25% dodecylmercaptan and 0.07% sodium hydroxide. The remaining space and theconnecting line is filled with styrene. The temperature is raised to 40C. and the circulating pump set in motion.

The circulation of the water phase in this case is also brought about bythe movement of the monomer drops. The styrene is originally introducedinto the water at the rate of about 10 liters per hour. After theconcentration of polymer in the water phase had risen to 1%, this rateis increased to 40 liters per hour.

The distribution of the droplet size of the monomer remains practicallyhomogeneous. The separation of the droplets at the bottom of the circuitproceeds rapidly and smoothly. The water phase circulates once in about10-20 sec.

After the concentration of polymer has risen to about 20%, the processis made continuous. By supplying fresh water phase and discharging thepolymer suspension formed, the concentration of polymer is kept at 20%.The styrene in the reservoir is replenished as required.

The polymer obtained by this process has excellent physical propertiesand is superior to a similar polymer prepared without circulating thewater phase.

Other polymers having improved properties may be obtained by replacingstyrene in the above-described process with each of the following:acrylonitrile, methacrylonitrile and vinyl chloride.

We claim as our invention:

1. In a process for producing a polymer of at least one substantiallywater-insoluble ethylenically unsaturated compound in an aqueous mediumwherein liquid drops of the said ethylenically unsaturated compound arepassed through a liquid aqueous medium whcih is maintained at apolymerizing temperature and contains a water-soluble polymerizationcatalyst and dispersed polymer of the ethylenically unsaturated compoundsuspended therein by the presence of sufiicient emulsifying agent thatthe polymer emulsion is substantially stable, and a portion of themonomer on the surface of the drops goes into the aqueous medium and ispolymerized, said passage of the drops of the ethylenically unsaturatedcompound through the aqueous medium being due primarily to thedifference in density of the said drops and medium and wherein all ofthe drops that pass through the aqueous phase are collected as aseparate phase in contact with the said aqueous medium, the improvementwhich comprises maintaining the space filled with the aqueous phase as aclosed circuit with the horizontal part of the circuit passing along theinterface of the aqueous phase and collected monomer phase wherein theaqueous phase is allowed ,to

continuously circulate around the .closed circuit in a substantiallyvertical plane. U

2. The improvement as defined in claim 1 wherein the distributing devicethrough which the monomer drops are introduced into the aqueous mediumis incorporated into the circuit.

3. The improvement as defined in claim 1 wherein the unsaturatedcompound is vinylidene chloride.

4. The improvement as defined in claim 1 wherein the speed of themonomer droplets through the aqueous medium exceeds the linear rate ofthe water phase by no more than 30 cm. per second.

5. The improvement as defined in claim 1 wherein the 12 circulation ofthe aqueous phase is due primarily to the movement of the monomerdroplets.

I '6; The improvement as defined in claim 1 wherein the unsaturatedcompound is vinyl chloride.

7. The improvement as defined in claim 1 wherein the unsaturatedcompound is styrene.

References Cited in the file of this patent UNITED STATES PATENTS2,179,040 Heuer Nov. 7, 1939 2,326,326 Breedis Aug. 10, 1943 2,566,567Hutchinson Sept. 4, 1951 2,618,626 Van Dijk et a1 Nov. 18, 1952

1. IN A PROCESS FOR PRODUCING A POLYMER OF AT LEAST ONE SUBSTANTIALLYWATER-INSOLUBLE ETHYLENICALLY UNSATURATED COMPOUND IN AN AQUEOUS MEDIUMWHEREIN LIQUID DROPS OF THE SAID ETHYLENICALLY UNSATURATED COMPOUND AREPASSED THROUGH A LIQUID AQUEOUS MEDIUM WHICH IS MAINTAINED AT APOLYMERIZING TEMPERATURE AND CONTAINS A WATER-SOLUBLE POLYMERIZATIONCATALYST AND DISPERSED POLYMER OF THE ETHYLENICALLY UNSATURATED COMPOUNDSUSPENDED THEREIN BY THE PRESENCE OF SUFFICIENT EMULSIFYING AGENT THATTHE POLYMER EMULSION IS SUBSTANTIALLY STABLE, AND A PORTION OF THEMONOMER ON THE SURFACE OF THE DROPS GOES INTO THE AQUEOUS MEDIUM AND ISPOLYMERIZED, SAID PASSAGE OF THE DROPS OF THE ETHYLENICALLY UNSATURATEDCOMPOUND THROUGH THEE AQUEOUS MEDIUM BEING DUE PRIMARILY TO THEDIFFERENCE IN DENSITY OF THE SAID DROPS AND MEIDUM AND WHEREIN ALL OFTHE DROPS THAT PASS THROUGH THE AQUEOUS PHASE ARE COLLECTED AS ASEPARATE PHASE IN CONTACT WITH THE SAID